TypeForm: Recognize type variables in a consistent way

Prerequisite: This issue applies after PR https://github.com/python/mypy/pull/19596 is merged.

Currently mypy sometimes allows a type variable to be assigned to a TypeForm variable and sometimes it does not. This is a problem because the behavior is inconsistent.

Specifically, mypy allows a type variable to be assigned to a TypeForm variable iff it is in a quoted type expression:

[case testTypeVarTypeFormsAreOnlyRecognizedInStringAnnotation]
...
E = TypeVar('E')
class Box(Generic[E]):
    def foo(self, e: E) -> None:
        list_of_typx: List[TypeForm] = [E]  # E: "E" is a type variable and only valid in type context
        typx1: TypeForm = E  # E: "E" is a type variable and only valid in type context
        typx2: TypeForm = 'E'

When a type variable is NOT quoted, mypy will not allow it to be assigned:

[case testEveryKindOfTypeExpressionIsAssignableToATypeFormVariable]
SomeTypeVar = TypeVar('SomeTypeVar')
typx: TypeForm
...
# NOTE: Unbound TypeVar isn't currently accepted as a TypeForm. Is that OK?
typx = SomeTypeVar  # E: Incompatible types in assignment (expression has type "TypeVar", variable has type "TypeForm[Any]")

I'd argue that TypeForm should always accept a type variable because it is a valid type expression and TypeForms are intended to accept all kinds of type expressions:

def cast[T](type_expr: TypeForm[T], value_expr: object) -> T: ...

cast(SomeTypeVar, some_expression)  # should not be an error

In summary:

• mypy should allow a type variable to be assigned to a TypeForm variable, regardless of whether it is quoted or not. Update above-referenced tests to reflect this new behavior.

まず前提となる PR #19596 を確認し、次に testTypeVarTypeFormsAreOnlyRecognizedInStringAnnotation と testEveryKindOfTypeExpressionIsAssignableToATypeFormVariable という名前のテストを見つけてください。TypeForm の代入で、引用符付きおよび引用符なしの型変数がどのように扱われるかを確認してください。両方のケースで一貫して型変数が許可され、テストが新しい動作を反映していれば完了です。